The configurable cells further comprise functional inputs, and when they do not receive a chaining command signal, the configurable cells are connected by their functional inputs and their outputs to logic cells with which they cooperate to form one or more logic circuits of the electronic circuit. This is a standard operating mode of the electronic circuit.
There are well-known procedures for verifying that the functional elements of an integrated circuit are working properly. These procedures entail the dictating and/or determining, at predefined points in time, of data values present at certain internal points of the integrated circuit.
A technique of this kind for testing internal test paths of an integrated circuit (called a scanpath or an internal scan method) is described for example by M. Williams and J. Angel, in “Enhancing Testability of LSI Circuits Via Test Points and Additional Logic,” IEEE Transactions on Computers, vol. C22, No. 1; January 1973. This technique and its advantages and drawbacks are also described in detail in European patent application no. 1,560,033, particularly paragraphs 2 to 16. The European patent application is assigned to the current assignee of the present invention, and is incorporated herein by reference in its entirety.
In addition to the internal scan method, the European patent application also describes an electronic circuit protected against fraud aimed at extracting data from the configurable cells. In the European patent application, when a change in the mode of operation is desired, the configurable cells are reset and a verification is made that they are properly reset before authorizing the change.
A drawback of this circuit is that any fraudulent person immediately realizes that the circuit is protected, and more specifically, realizes this as soon as they ask for a change in the mode. A fraudulent individual, whose obvious goal is to extract data from the test register, immediately realizes that the test register does not contain information. Since the cells are reset with the effective change in the mode, the data likely to be extracted is identical at each change in the mode.
The efficiency of a procedure for protecting an electronic circuit is particularly measured by taking into account the time that a possible fraudulent individual will take to determine that protection exists before seeking a way to circumvent the protection.